<p>The topic of this thesis is signaling design for data transmission through wireless channels between a transmitter and a receiver that can both be equipped with one or more antennas. In particular, the focus is on channels where the propagation coefficients between each transmitter--receiver antenna pair are only partially known or completetly unknown to the receiver and unknown to the transmitter.</p><p>A standard signal design approach for this scenario is based on separate training for the acquisition of channel knowledge at the receiver and subsequent error-control coding for data detection over channels that are known or at least approximately known at the receiver. If the number of parameters to estimate in the acquisition phase is high as, e.g., in a frequency-selective multiple-input multiple-output channel, the required amount of training symbols can be substantial. It is therefore of interest to study signaling schemes that minimize the overhead of training or avoid a training sequence altogether.</p><p>Several approaches for the design of such schemes are considered in this thesis. Two different design methods are investigated based on a signal representation in the time domain. In the first approach, the symbol alphabet is preselected, the design problem is formulated as an integer optimization problem and solutions are found using simulated annealing. The second design method is targeted towards general complex-valued signaling and applies a constrained gradient-search algorithm. Both approaches result in signaling schemes with excellent detection performance, albeit at the cost of significant complexity requirements.</p><p>A third approach is based on a signal representation in the frequency domain. A low-complexity signaling scheme performing differential space--frequency modulation and detection is described, analyzed in detail and evaluated by simulation examples.</p><p>The mentioned design approaches assumed that the receiver has no knowledge about the value of the channel coefficients. However, we also investigate a scenario where the receiver has access to an estimate of the channel coefficients with known error statistics. In the case of a frequency-flat fading channel, a design criterion allowing for a smooth transition between the corresponding criteria for known and unknown channel is derived and used to design signaling schemes matched to the quality of the channel estimate. In particular, a constellation design is proposed that offers a high level of flexibility to accomodate various levels of channel knowledge at the receiver.</p>

Η εργασία αυτή ασχολείται με το πρόβλημα της εκτίμησης καναλιού σε συστήματα μετάδοσης OFDM. Το πρόβλημα αυτό συγκεντρώνει έντονο ερευνητικό ενδιαφέρον τα τελευταία χρόνια, καθώς συναντάται σε ένα ευρύ πεδίο εφαρμογών. Η άγνωστη συνάρτηση μεταφοράς του καναλιού στα ασύρματα συστήματα μετάδοσης, καθιστά απαραίτητη την εκτίμησή του πριν από οποιαδήποτε διαδικασία μετάδοσης. Στη συγκεκριμένη μεταπτυχιακή εργασία, αντικείμενο εξέτασης αποτελεί η επίδραση καναλιού με μη γραμμικά χαρακτηριστικά σε συστήματα μετάδοσης OFDM. Αρχικά, παρουσιάζεται ένας βελτιωμένος αλγόριθμος εκτίμησης καναλιού, ο οποίος βασίζεται σε ένα παραμετρικό μοντέλο. Η απόκριση συχνότητας του καναλιού εκτιμάται χρησιμοποιώντας ένα μοντέλο με L μονοπάτια. Γίνεται χρήση της μεθόδου ESPRIT για την αρχική εκτίμηση των πολυδρομικών καθυστερήσεων, ενώ η διαδικασία παρακολούθησης γίνεται με την τεχνική IPIC DLL. Με γνωστή την πληροφορία για τις πολυδρομικές καθυστερήσεις, εκτιμάται η απόκριση του καναλιού στο πεδίο της συχνότητας με τη μέθοδο του ελαχίστου μέσου τετραγωνικού σφάλματος. Ιδιαίτερης μνείας χρήζει το κριτήριο MDL (Minimum Description Length) που χρησιμοποιείται για την εύρεση των ενεργών μονοπατιών του καναλιού. Σύμφωνα με το κριτήριο, υπολογίζεται ο ιδιοχώρος, δηλαδή οι ιδιοτιμές και τα ιδιοδιανύσματα, του πίνακα αυτοσυσχέτισης του καναλιού. Ο ιδιοχώρος αυτός εμφανίζει ιδιαίτερη δομή και μπορεί να αναλυθεί σε κάθετους μεταξύ τους υποχώρους: τον υποχώρο του σήματος (signal subspace) και αυτόν του θορύβου (noise subspace). Έχει αποδειχθεί ότι η χρήση παραμετρικού μοντέλου καναλιού μπορεί να μειώσει δραστικά τις διαστάσεις του υποχώρου του σήματος και κατά συνέπεια να βελτιώσει την απόδοση της εκτίμησης του καναλιού. Στη συνέχεια εξετάζεται η δυνατότητα εφαρμογής του αλγόριθμου PAST κατά τη διαδικασία παρακολούθησης των πολυδρομικών καθυστερήσεων και η σύγκρισή του με την απόδοση του IPIC DLL. Ο αλγόριθμος PAST έχει χαμηλή υπολογιστική πολυπλοκότητα καθώς στηρίζεται σε αναδρομικές τεχνικές παρακολούθησης του ιδιοχώρου. Στα πλαίσια της μεταπτυχιακής εργασίας έγινε συγκριτική μελέτη των τεχνικών εκτίμησης καναλιού σε συστήματα μετάδοσης OFDM. Περιγράφονται τα βασικά χαρακτηριστικά των κυριότερων αλγορίθμων της βιβλιογραφίας και στη συνέχεια παρουσιάζονται τα αποτελέσματα των προσομοιώσεων σε περιβάλλον MATLAB. Με βάση τη θεωρητική μελέτη των μεθόδων εκτίμησης και τα αποτελέσματα των προσομοιώσεων, εξάγονται συμπεράσματα για τη βελτίωση της απόδοσης που παρουσιάζουν σε σχέση με τις μη παραμετρικές τεχνικές. Τέλος, υλοποιήθηκε ένας νέος αλγόριθμος για την εύρεση του υποχώρου του σήματος, ο οποίος βελτιώνει σημαντικά την απόδοση του κριτηρίου MDL. / The basic concept in this thesis is the problem of Channel Estimation in multipath fading chanels. The method we use is based on parametric channel modeling. Firstly, we use the MDL (Minimum Descreption Length) criterium in order to estimate the number of paths in the channel. Next the ESPRIT method calculates the Time Delays for every estimated path. The second part of the algorithm is used for tracking of time delays. We firstly use an IPIC DLL (InterPath Interference Cancellation Delay Locked Loop) technique and then the path gains are calculated via a MMSE estimator. There is also a study in Subspace Tracking problem. We use the PAST and PASTd algorithms to calculate the signal subspace for every OFDM symbol transmited. The two techniques we described increase the SER performance of the non parametric channel estimator by 2dB and the MSE performance by 5dB. We also describe a new algorithm which has better performance than the MDL criterium.

To date the majority of research in the area of cooperative communications focuses on maximizing throughput and reliability while assuming perfect channel state information (CSI) and synchronization. This thesis, seeks to address performance enhancement and system parameter estimation in cooperative networks while relaxing these idealized assumptions. In Chapter 3 the thesis mainly focuses on training-based channel estimation in multi-relay cooperative networks. Channel estimators that are capable of determining the overall channel gains from source to destination antennas are derived. Next, a new low feedback and low complexity scheme is proposed that allows for the coherent combining of signals from multiple relays. Numerical and simulation results show that the combination of the proposed channel estimators and optimization algorithm result in significant performance gains. As communication systems are greatly affected by synchronization parameters, in Chapter 4 the thesis quantitatively analyzes the effects of timing and frequency offset on the performance of communications systems. The modified Cramer-Rao lower bound (MCRLB) undergoing functional transformation, is derived and applied to determine lower bounds on the estimation of signal pulse amplitude and signal-to-noise ratio (SNR) due to timing offset and frequency offset, respectively. In addition, it is shown that estimation of timing and frequency offset can be decoupled in most practical settings. The distributed nature of cooperative relay networks may result in multiple timing and frequency offsets. Chapters 5 and 6 address multiple timing and frequency offset estimation using periodically inserted training sequences in cooperative networks with maximum frequency reuse, i.e., space-division multiple access (SDMA) networks. New closed-form expressions for the Cramer-Rao lower bound (CRLB) for multiple timing and multiple frequency offset estimation for different cooperative protocols are derived. The CRLBs are then applied in a novel way to formulate training sequence design guidelines and determine the effect of network protocol and topology on synchronization parameter estimation. Next, computationally efficient estimators are proposed. Numerical results show that the proposed estimators outperform existing algorithms and reach or approach the CRLB at mid-to-high SNR. When applied to system compensation, simulation results show that application of the proposed estimators allow for synchronized cooperation amongst the nodes within the network. / Thesis (Ph.D, Electrical & Computer Engineering) -- Queen's University, 2010-07-29 16:52:50.272

This research investigates techniques for iterative channel estimation to maximize channel capacity and communication security. The contributions of this dissertation are as follows: i) An accurate, low-complexity approach to pilot-assisted fast-fading channel estimation for single-carrier modulation with a turbo equalizer and a decoder is proposed. The channel is estimated using a Kalman filter (KF) followed by a zero-phase filter (ZPF) as a smoother. The combination of the ZPF with the KF of the channel estimator makes it possible to reduce the estimation error to near the Wiener bound. ii) A new semi-blind channel estimation technique is introduced for multiple-input-multiple-output channels. Once the channel is estimated using a few pilots, a low-order KF is employed to progressively predict the channel gains for the upcoming blocks. iii) The capacity of radio channels is investigated when iterative channel estimation, data detection, and decoding are employed. By taking the uncertainty in decoded data bits into account, the channel Linear Minimum Mean Square Error (LMMSE) estimator of an iterative receiver with a given pilot ratio is obtained. The derived error value is then used to derive a bound on capacity. It is shown that in slow fading channels, iterative processing provides only a marginal advantage over non-iterative approach to channel estimation. Knowing the capacity gain from iterative processing versus purely pilot-based channel estimation helps a designer to compare the performance of an iterative receiver against a non-iterative one and select the best balance between performance and cost. iv) A Radio channel is characterized by random parameters which can be used to generate shared secret keys by the communicating parties when the channel is estimated. This research studies upper bounds on the rate of the secret keys extractable from iteratively estimated channels. Various realistic scenarios are considered where the transmission is half-duplex and/or the channel is sampled under the Nyquist rate. The effect of channel sampling interval, fading rate and noise on the key rate is demonstrated. The results of this research can be beneficial for the design and analysis of reliable and secure mobile wireless systems. / Graduate / 0544

In this thesis, various pilot symbol aided channel estimation and tracking methods are investigated and their performances are compared for an OFDM system with packet based communication on HF channel. For the HF channel, Watterson HF channel model is used. The compared methods are least squares (LS) channel estimation, linear minimum mean square error (LMMSE) channel estimation, least mean squares (LMS) channel tracking, recursive least squares (RLS) channel tracking, constant position model based Kalman filter channel tracking, and constant velocity model based Kalman filter channel tracking. For LMS and RLS methods some adaptive approaches are also investigated.

Terrestrial Digital Video Broadcasting (DVB-T) is a standard for wireless broadcast of MPEG-2 video. DVB-T is based on channel coding algorithms and uses Orthogonal Frequency Division Multiplexing (OFDM) as a modulation scheme. In this thesis, we have implemented the standard of ETSI EN 300 744 for Digital Video Broadcasting in MATLAB. This system is composed of the certain blocks which include OFDM modulation, channel estimation, channel equalization, frame synchronization, error-protection coding, to name a few of such blocks. We have investigated the performance of the complete system for different wireless broadcast impairments. In this performance analysis, we have considered Rayleigh fading multi-path channels with Doppler shift and framing synchronization errors and obtained the bit error rate (BER), and channel minimum square error performances versus different maximum Doppler shift values, different channel equalization techniques and different channel estimation algorithms. Furthermore, we have investigated different interpolations methods for the interpolation of channel response. It is shown that minimum mean-square error (MMSE) type equalization has a better performance in symbol estimation compared to zero forcing (ZF) equalizer. Also linear interpolation in time and low pass frequency interpolation, for time frequency interpolation of channel response can be used for practical application.

Interference and jamming severely disrupt our ability to communicate by decreasing the effective signal-to-noise ratio and by making parameter estimation difficult at the receiver. The objective of this research work is to design robust wireless systems and algorithms to suppress the adverse effects of non-intentional co-channel interference (CCI) or intentional jamming. In particular, we develop chip-combining schemes with timing, channel, and noise-power estimation techniques, all of which mitigate CCI or jamming. We also exploit the spatial diversity and iterative receiver techniques for this purpose. Most of the existing timing estimation algorithms are robust against either large frequency offsets or CCI, but not against both at the same time. Hence, we develop a new frame boundary estimation method that is robust in the presence of severe co-channel interference and large carrier-frequency offsets. To solve the high peak-to-average-power ratio problem of a multicarrier code division multiple access (MC-CDMA) system and enhance its robustness against fading and jamming, we propose a constant-envelope MC-CDMA system employing cyclic delay diversity (CDD) as transmit diversity. We analyze the diversity order, coding gain, and bit-error rate upper bound. We also propose a blind, accurate, and computationally efficient signal-to-noise ratio estimator for the proposed system. We propose a configurable robust anti-jam receiver that estimates the frequency- or time-domain jammer state information (JSI) and uses it for chip combining in the corresponding domain. A soft-JSI-based chip-combining technique is proposed that outperforms conventional hard-JSI-based chip combining. We also derive a chip combiner that provides sufficient statistics to the decoder. Channel estimation is necessary for coherent signal detection and JSI estimation. Conversely, knowledge of the jamming signal power and JSI of different subcarriers can improve the accuracy of the channel estimates. Hence, we propose joint iterative estimation of the multiple-input multiple-output (MIMO) channel coefficients, jamming power, and JSI for a coded MC-CDMA MIMO system operating under jamming and a time-varying frequency-selective fading channel. Finally, we reduce the computational complexity of the JSI-based anti-jam receivers by introducing an expectation-maximization-based joint channel and noise-covariance estimator that does not need either the subcarrier JSI or the individual powers of the AWGN and jamming signal.

Orientador: Prof. Dr. Murilo Bellezoni Loiola / Dissertação (mestrado) - Universidade Federal do ABC, Programa de Pós-Graduação em Engenharia da Informação, 2017.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / In this master thesis it is proposed a MIMO communication scheme with four transmit antennas and unitary transmission rate. The performance and robustness of the scheme are evaluated and compared with other good proposals recently presented in the literature. The proposed scheme uses a preprocessor based on phase feedback. The preprocessor allows the proposed scheme to obtain full diversity and also coding gain. The pre-processing considers a codebook design, whose size is dependent on the number of feedback bits. Error probability analysis is provided, where the upper and lower bounds for different numbers of feedback bits and antennas are presented, considering two different types of constellations, QAM and PSK. For robustness analysis, channels estimators were used. For the channel estimators model it was added the effects of spatial correlation, enabling the evaluation the losses caused by the spatial correlation among the antennas, and the Doppler effect, for evaluating the loss performance due to the relative mobilitys between trnamitter and receiver. / Nesta dissertação propõe-se um esquema de comunicação MIMO para quatro antenas transmissoras com taxa de transmissão unitária. O desempenho e a robustez do esquema proposto são avaliados e comparados com outras boas propostas recentemente apresentadas na literatura. O esquema proposto utiliza um pré-processador baseado na realimentação de fase provinda do receptor. O pré-processamento permite que o esquema proposto alcance um grau de diversidade completo bem como um ganho de codificação. A pré-codificação considera o uso de um codebook, cujo comprimento depende do número de bits de realimentação disponível. Faz-se uma análise de desempenho da probabilidade de erro, mostrando os limitantes superior e inferior do esquema para diferentes quantidades de bits de realimentação, diferentes números de antenas e para as constelações do tipo PSK e QAM. Para a análise de robustez, considerou-se o emprego de estimadores de canais clássicos. No modelo utilizado para os estimadores de canais foram acrescentados os efeitos da correlação espacial, para se verificar as perdas decorrentes da correlação espacial entre as antenas, e do efeito Doppler, para se avaliar a perda de desempenho decorrente da mobilidade relativa entre transmissor e receptor.

La mise en œuvre de systèmes basés OFDMA nécessite une synchronisation de la fréquence très fine en raison de l'extrême sensibilité de l'OFDMA aux décalages en fréquence porteuse (CFO). La synchronisation en fréquence devient plus difficile dans les systèmes OFDMA en liaison montante. Nos objectifs comprennent l'étude et l'analyse des problèmes résultant de décalages de fréquence et la proposition de solutions pour lutter contre ces problèmes. Nous examinons d'abord les interférences résultant de décalages de fréquence porteuse présents dans l'oscillateur du terminal utilisateur. Nous démontrons que l'on doit prendre en compte le préfixe cyclique tout en analysant les interférences résultant du CFO. Ensuite, nous montrons qu'il existe une contradiction entre la diversité de fréquence de canal et la robustesse contre le CFO. Nous proposons un compromis sous la forme d'une taille de bloc de seuil, afin de permettre un bon compromis entre la diversité des canaux et de robustesse pour les CFO pour le cas où aucune connaissances du canal n'est disponible. Quand le canal est connu, nous proposons une allocation optimale par bloc grâce à laquelle la robustesse aux CFO et à la diversité de fréquence de canal peut être réalisée en utilisant une petite taille de bloc petit pour des CFO de petite valeur. Nous proposons également une valeur CFO critique, en dessous de laquelle l'allocation optimal par bloc est très performant.Ensuite, nous proposons des solutions pour deux problèmes importants rencontrés dans un système OFDMA en liaison montante.Premièrement, nous proposons une méthode efficace pour l'estimation conjointe des réponses impulsionnelles des canaux et fréquences porteuses basée sur l'approximation polynomiale.Notre méthode d'estimation conjointe est plus simple que les méthodes existantes, sans aucune dégradation de performance. Ensuite, nous proposons une méthode de compensation de CFO basée sur l'annulation des interférences successives (SIC). La méthode d'annulation proposée réduit la complexité de mise en oeuvre quand le nombre de porteuses est important. / One of the most prominent issues in the design and implementation of OFDMA based systems is the need for a very fine frequency synchronization due to the fact that OFDMA, like OFDM, is extremely sensitive to carrier frequency offsets (CFO). The task of frequency synchronization becomes more challenging in the uplink OFDMA-based systems where one OFDMA symbol is generated by the contribution of many different users. Our goals include the study and analysis of problems resulting from frequency mismatches provide solution to combat these problems. We first look at the interference resulting from CFOs, resulting from user terminal oscillator mismatch. We demonstrate that one must take into account the cyclic prefix while analyzing interference resulting from CFO. A new analytical expression of the ICI that takes into account the effect of CFO on the cyclic prefix is proposed. Then we focus our attention on analysis of the trade-off between channel frequency diversity and robustness against CFO and show that there exists a contradiction between the two. We propose a trade-off in the form of a Threshold blocksize, to allow a good compromise between the channel diversity and robustness for CFO for the case when no CSI is available. For system where CSI is available, we propose an optimal block carrier allocation scheme through which both robustness to CFO and channel frequency diversity can be achieved with small blocksize for small CFO. We also propose a Critical CFO value, above which the performance of the optimal block carrier allocation loses interest.Next we propose solutions for two important issues encountered in an uplink OFDMA system. First, we propose an efficient method for joint estimation of channel impulse responses and carrier frequency at the receiver based on polynomial approximation. Our proposed joint estimation method is simpler than the existing methods without any performance degradation. Next we propose a CFO compensation method based on successive interference cancellation. The proposed cancellation method reduces the implementation complexity faced in case of large DFT matrices.